I. Introduction
This invention relates to microlithography, and more particularly, to a process of microlithography that uses a photoresist coating protected by a metal mask. Specifically, the invention relates to the formation of a patterned metal mask over a photoresist coating, and the use of the mask to dry develop the photoresist coating and/or transfer an image defined by a developed photoresist coating to an underlying substrate.
II. Discussion of Related Art
Recent advances in electronic device fabrication have resulted from improvements in manufacturing techniques, especially improvements in microlithographic methods and methods for transferring patterns from a master to a substrate utilizing a patterned photoresist. For fine line patterning, anisotropic dry development with a reactive ion etch is preferred to wet development because anisotropic etching results in a relief image having vertical side walls. This permits transfer of an image of enhanced resolution to an underlying substrate.
Photoresists used in processes of the above type are organic polymers often containing a separate light sensitive component. Contact of an organic coating with an oxygen plasma causes erosion or ablation of all surfaces of the coating contacted by the plasma stream. For this reason, to create an image pattern in a photoresist coating by dry development with a plasma, the photoresist coating must have areas that are plasma resistant and other areas that are not plasma resistant, the plasma resistant areas and the areas subject to attack by the plasma defining the desired image pattern.
Methods for dry development of photoresist coatings in an image pattern are known in the art. For example, a known method for formation of a plasma resistant photoresist coating in an image pattern involves use of a photoresist containing silicon as part of the photoresist composition in sufficient quantity whereby following exposure to patterned activating radiation and upon contact with a plasma, a silicon oxide mask is formed over the surface of the photoresist, the silicon oxide mask functioning as a patterned protective layer that limits erosion of those portions of the photoresist coating underlying the silicon oxide mask.
A method for generating a pattern in a photoresist coating using a technique similar to that described above is disclosed in U.S. Pat. No. 4,426,247. The method comprises the steps of forming a polymer layer on a substrate, forming a silicon layer on the polymer layer, selectively irradiating a surface of the silicon layer with a high energy beam, exposing the surface of the silicon layer to a radical addition polymerizable monomer gas so as to form a graft polymer film as a mask on an irradiated portion of the surface of the silicon layer, performing reactive ion etching using the graft polymer film as a mask so as to form a silicon pattern, and a reactive ion etching using the silicon pattern as a mask to protect underlying organic polymer layers, so as to form an organic polymer pattern.
More recent processes have been developed that permit selective conversion of portions of a nonsilicon containing photoresist to a silicon containing etch resistant mask. In these methods, the nonsilicon containing photoresist is exposed to patterned radiation to create a latent image within the photoresist. The latent image is then reacted with an organometallic reagent to incorporate an oxide forming element such as a metal or silicon into the image. The metallized or siliconized latent image is then dry developed, and etch resistant images, as well as underlying organic material, if any, can be dry developed using a suitable plasma to sequentially develop and transfer the pattern to a substrate below. Such methods are disclosed in U.S. Pat. No. 4,613,398. As disclosed in that patent, the metallic portion of the organometallic material can be selected from the Group III A metals, Group IV A metals, Group IV B metals, and Group VI B metals. The preferred elemental portions are disclosed as titanium, silicon and tin, with the most preferred being silicon.
Though the methods described above yield etch resistant photoresist layers, results are not readily reproducible and the processes are cumbersome, time consuming and costly. Therefore, there is a continuing need for an improved process for providing dry etch resistant masks over photoresists for transfer of micron and submicron images to a substrate.